1. Field of the Invention
This invention relates to the field of metal part fabrication, and particularly to the fabrication of parts from metal powder.
2. Description of the Related Art
The use of a metal powder to manufacture metal parts is well-known. Typically, a metal powder and a lubricant are tightly compacted into a mold or a die, forming a xe2x80x9cgreen formxe2x80x9d part. Green form parts resemble the desired part, but lack density and strength. The fragile green form part is subjected to a heating step, which sinters the metal powder and causes the part to densify and consolidate.
U.S. Pat. No. 5,745,834 to Bampton et al. and assigned to the present assignee (xe2x80x9cthe ""834 patentxe2x80x9d) describes a xe2x80x9cfree formxe2x80x9d fabrication method, in which a green form part is built up layer-by-layer by localized laser melting of a powder blend that includes both metal and a polymer binder. The green form part then undergoes densification in the conventional manner.
A method used to fabricate metal parts having surface features only is described in co-pending patent application Ser. No. 09/404,227 (assigned to the present assignee). Here, a powder blend that includes both metal and a polymer binder is poured into a mold, which is heated to form the green form part. After removing the green part from the mold, it is placed in an oven to be consolidated.
The methods described above are capable of producing a wide variety of metal parts. However, one drawback common to all of these techniques is the limited size of the parts that can be made. Green form parts having thin walls or complex structures can lose their strength when subjected to the heat required for consolidation. This is due to the fact that the lubricant or polymer binder used in the powder blend tends to melt at the elevated consolidation temperature, becoming a low viscosity fluid that weakens the structure. The weakness can result in the structure failing during consolidation, particularly if it is large or has unsupported members extending from it.
A method of strengthening green form parts made from metal powder is presented, which enables the fabrication of larger powder-based parts than has heretofore been possible.
The novel method requires the use of a polymer which retains its mechanical properties to a degree sufficient to prevent fracture or significant deformation of the green form part when subjected to heat sufficient to induce phase transformation and, carbonization. Incorporating such a polymer into a green form part prior to densification gives it additional strength, which enables the part to endure the densification temperature without failing.
Cross-linkable polymers provide the mechanical properties described above, and as such are specified for use in the present method. Cross-linking is induced once the polymer is incorporated into the green part. The polymer may be incorporated into the green form part in a number of ways. For example, the cross-linkable polymer can be part of the powder blend used to form the green form part, and induced to cross-link when subjected to the heat of consolidation, or to a particular type of radiation. Another possibility requires the green form part to be dipped in a thermoset resin that cross-links when cured. Regardless of the method used to incorporate the cross-linkable polymer, once cross-linking has occurred, it retains more of its mechanical properties when subjected to the high heat of densification than it would otherwise. This lends strength to the structure and enables it to endure the consolidation process, thus overcoming the shortcomings of prior art fabrication methods.
Further features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings.